Examples of capacitors used in electronic apparatuses include ceramic capacitors, tantalum electrolytic capacitors, aluminum electrolytic capacitors, and film capacitors. Although there are single-layer ceramic capacitors which are constructed of a pair of electrodes, most ceramic capacitors are constructed of laminated ceramic capacitors having a structure in which a plurality of electrode pairs is laminated. The ceramic capacitors can be manufactured in a small size because of their dielectrics having high permittivity and are widely used in modern electronic apparatuses because of their capacitances exhibiting small changes with aging. The ceramic materials used in ceramic capacitors exhibit ferroelectric properties and so-called piezoelectric or electrostriction such that the ceramic materials are mechanically deformed when an electric voltage is applied to both ends of the capacitor.
In ceramic capacitors, high-permittivity ferroelectrics, which are mainly made of barium titanate (BaTiO3), are typically used. Such ferroelectrics have a perovskite crystal structure, so that, in their sintered state, spontaneous polarization occurs within them and crystal grains are oriented in random directions. Therefore, even when an external electric field is applied to the ferroelectrics being in such a state, piezoelectric effects of the respective crystal grains cancel out, whereby the ferroelectrics as a whole do not exhibit piezoelectric properties. When a high DC voltage is applied to the ferroelectrics, however, the respective crystal grains are polarized in the direction of the electric field, thereby exhibiting piezoelectric properties.
A DC-DC converter mounted on a computer supplies electric power to electronic devices by converting an input DC voltage into predetermined DC voltages appropriate for the respective electronic devices. In an input/output circuit of the DC-DC converter, a ripple voltage of an audible frequency range is generated due to its switching operation in the form of being superimposed on the DC voltage. A central processing unit (CPU) is also connected to the DC-DC converter as a load. When the computer is equipped with a UHCI_USB (Universal Host Controller Interface—Universal Serial Bus) master, the CPU transitions between C2 and C4 states every 1 ms in order to perform a power saving operation, and this transition appears as a ripple voltage of 1 KHz to the input/output circuit of the DC-DC converter.
The input/output circuit of the DC-DC converter or an IC chip is connected to a decoupling capacitor in order to reduce noise based on the switching operation. When the voltage of an audible frequency range superimposed on the DC voltage is applied to the decoupling capacitor, the capacitor exhibits a reverse piezoelectric effect in response to an alternating component of the electric field, so that the capacitor's body resonates with its natural frequency and vibrates. Therefore, when laminated ceramic capacitors are mounted on a printed circuit board (PCB) as decoupling capacitors, the capacitors' vibrations are transmitted to the PCB and generate noise.
Japanese Patent Application Laid-Open No. 2002-232110 teaches a technique for reducing vibration noise which is generated when vibrations of first and second ceramic capacitors generated due to a ripple component based on their charging/discharging operations are transmitted to a circuit board. The first and second ceramic capacitors are arranged so as to be aligned on one surface side of the circuit board, and the vibration waves transmitted to the circuit board have approximately equal amplitudes and opposite phases on one surface side of the circuit board.
Japanese Patent Application Laid-Open No. 2004-273935 teaches a ceramic capacitor having a structure capable of suppressing transmission of vibration to a circuit board. The ceramic capacitor element includes a dielectric base, a pair of terminal electrodes, and a plurality of internal electrodes. The pair of terminal electrodes is formed at opposite lateral ends of the dielectric base, and each of the plurality of internal electrodes has one end thereof being connected to the terminal electrode and the other end thereof being left as an open end. A pair of metal terminals has a respective board mounting portion and is connected to a corresponding terminal electrode. The board mounting portions are disposed on one mounting surface, and the mounting surface intersects the electrode face of the internal electrodes at approximately right angles. Owing to such a construction, the vibration transmitted via the metal terminals to the board can be reduced even when the ceramic capacitor element vibrates due to electrostriction phenomenon.
Japanese Patent Application Laid-Open No. 2003-324030 teaches a method of mounting a laminated ceramic capacitor on a circuit board capable of reducing noise generated due to piezoelectric phenomenon. In this method, two laminated ceramic capacitors connected in series are mounted at plane symmetry positions on top and back surfaces of a circuit board. According to this method, since two capacitors are connected in series, vibrations transmitted from respective capacitors to the circuit board cancel out, whereby the circuit board is prevented from resonating.